1. Field of the Invention
The present invention relates to a transmission-type screen and a transmission-type video projection television system which employs such a transmission-type screen for displaying an image projected from a rear projector for visual observation by an observer in front of the screen, and more particularly to an uncharged transmission-type screen which has an electrically conductive surface to prevent the screen from being electrostatically charged for thereby preventing dust or dirt particles in the air from being attracted to the screen, and a video projection television system which employs such an uncharged transmission-type screen.
2. Description of the Prior Art
One conventional transmission-type screen of plastic material for use in transmission-type video projection television is of a double-lens structure comprising a Fresnel lens and a lenticular lens which are combined with each other. The transmission-type screen has black stripes on its surfaces for absorbing and blocking rays of light. The black stripes are typically printed with black ink or coated with black paint. The black stripes of blank ink or paint present an electric resistance of 1.4.times.10.sup.11 .OMEGA. on the exit-side surface (a) of the lenticular lens, an electric resistance of 3.5.times.10.sup.11 .OMEGA. on the entrance-side surface (b) of the lenticular lens, an electric resistance of 1.4.times.10.sup.11 .OMEGA. on the exit-side surface (c) of the Fresnel lens, and an electric resistance of 1.4.times.10.sup.11 .OMEGA. on the exit-side surface (d) of the lenticular lens, an electric resistance of 1.4.times.10.sup.11 .OMEGA. on the entrance-side surface of the lenticular lens, as shown in FIG. 18 in the row indicated at .alpha.. Since these surface electric resistances are very high, the screen surfaces can easily be triboelectrically charged up to an extremely high electrostatic potential. For example, when the exit-side surface (a) of the lenticular lens is rubbed, it is electrostatically charged up to 10 KV; and when the entrance-side surface (d) of the Fresnel lens is rubbed, it is electrostatically charged up to 30 KV. The electrostatically charged screen surfaces easily attract dust or dirt particles in the ambient air, and can be smeared very quickly.
When the dust or dirt deposit is wiped off the charged screen surfaces with a dust cloth, the other screen surfaces, which are opposite the charged screen surfaces, also become triboelectrically charged up to a high electrostatic potential.
In order to prevent the screen surfaces from being triboelectrically charged, it has been customary to use a surface-active agent as an antistatic agent which does not erode the plastic material.
However, surface-active agents have a short service life, and remain effective for only half through one year even when they are left unused. When dust or dirt deposits are wiped off surfaces that are coated with a surface-active agent, the surface-active agent is also gradually wiped away, and hence has a shorter service life.
The surface-active agent which is applied to the entire surfaces of a transmission-type screen of plastic material is selected to be transparent and highly transmission of light. Since the surface-active agent is heat-resistant only at low temperatures ranging from 60.degree. to 80.degree. C., it must be treated at a low temperature and does not last for a long time.
When deposited dust or dirt particles are wiped off the screen, the surface-active agent is also removed from the screen by the friction within a short period of time. Therefore, the screen tends to attract dust or dirt particles easily under electrostatic charges that are developed by triboelectricity. This in turn requires the screen to be wiped again to get rid of the attracted dust or dirt particles, resulting in more triboelectric attraction of dust or dirt particles.